12
Appendix "B"
– ABOUT THE CABLE
ABOUT THE CABLE TO BE USED
Recommendations for the cable to be used with the 2 wired systems:
Never mix in one cable two lines of separate 2w channels.
Never mix in the same cable a 2 wired channel and an RS485 communication line.
Always maintain a distance of at least 20 cm between the cables of different 2W
channels and RS485 when laid in the ground.
The cable should be type NYY, which is double, coated and is suitable for being laid
underground.
Thickness 1.5 mm2
The cable capacity is of great importance, the lower the better. A capacity of 0.1 µF
per km is OK. The total capacity of the cables connected to the "interface 2W" should
not exceed 1 µF.
The resistance of the cable should be reasonably low, with no leakage between the
wires and between each wire to the ground. (see testing below).
The cable used for the "2 wired" system should not be used for other purposes such
as communication between the DREAM and other interfaces or between the DREAM
and the PC.
CABLE RESISTANCE TESTING
1. The cable must be tested for continuity of its wires, and for having good isolation
between the wires and between each wire and the ground.
2. Disconnect both ends of the cable under test (including from any RTU in the middle)
and make sure the wires are not touching each other.
3. Check the resistance between the wires. Use the highest range available on the
ohmmeter (tens or hundreds of K
Ω). The resistance should be infinite or at least not
lower than 1 M
Ω.
4. Check the resistance between each wire to the ground Use the highest range
available on the ohmmeter (tens or hundreds of K
Ω). The resistance should be
infinite or at least not lower than 1 M
Ω.
5. Make a short circuit between the wires at one end of the cable and test the resistance
between the wires at the other end. This time use the lowest range of your ohmmeter
(tens or hundreds of
Ω). The resistance between a pair of wires increases with the
length of the wires and decreases with their thickness. For 1 Km distance and with a
pair of wires with 1.5 mm
2
cross section the resistance should be about 22
Ω. The
formula for calculating the expected resistance of a pair of copper wires is the
following:
Resistance (in
Ω) = 0.017 x Length (in meters)
Cross section (in mm
2
)
